1. Field of the Invention
The present invention relates to devices for measuring characteristic absorption of fluid samples or gas samples. More particularly, the present invention relates to a spectrophotometric measurement cell that also pumps fluid samples into and out of the cell.
2. Discussion of Background
Light absorption cells that measure the characteristic absorption spectrum in a fluid sample are well known in the prior art. A typical spectrophotometer measurement system includes a light source, a sample cell, and a detector. The sample cell usually contains a lens to direct the light from the light source through the sample fluid, and a second lens to direct light coming from the sample fluid to the detector. The absorption spectrum from the light passing through the sample cell is measured by the detector. Often, spectrophotometric measurements are taken from a known or reference sample, such as distilled water, and then compared to measurements taken from the fluid sample. Comparison with the known or reference sample assists in the determination of the concentraions of various components in the fluid sample.
Several spectrophotometric measuring devices have fixed distances between the light source and the detector. For instance, Topol et al., U.S. Pat. No. 4,451,152, and Farr, U.S. Pat. No. 3,142,719, disclose measuring chambers with a light source and a detector on opposing sides of the chamber, a fixed distance apart. The fluid sample is drawn into the chamber by a piston, and the intensity of the light transmitted through the sample is measured.
It is often desirable to have the measurement capability of varying the optical path length. Johnson, U.S. Pat. No. 4,488,814, and Jayko U.S. Pat. No. 3,448,277, disclose devices that vary the optical path length by changing the distance between the light source and the detector. Jayko uses a syringe tube configuration to enable a plunger to vary the optical path length between the light source and the detector. The syringe tube also draws the fluid sample into the measuring environment between the light source and the detector. However, neither of these devices appears to be suitable for on-line use.
Heigl, et al., U.S. Pat. No. 3,740,156, discloses a photometric sampling cell device for on-line analysis of process streams. The sample cell, which is positioned within the process stream, contains two windows, one of which can be manually adjusted a predetermined distance from the other window while in the process stream. The fluid sample is located within the sample cell between the two windows. Measurement can then be made as light from a conventional source passes through one of the windows, through the sample, then through the other window to a receiver. However, the adjustment of the windows is manual and cannot readily be done while measurements are in progress.
A probe for making remote optical absorption measurements is disclosed by O'Rourke, et al., U.S. Pat. No. 5,039,224. The probe allows variable optical path length by using a sliding, reflecting plug to reflect light coming from the light source through the fluid sample and back to a detector. The mechanical movement of the sliding plug allows the optical path length to be varied remotely and while spectral measurements are in progress. The probe has ports for admitting and expelling the fluid sample.
Despite the existence of numerous spectrophotometric measuring devices, it is believed that no device uses a portion of the measurement means to vary the optical path length and also to pump the fluid sample into and out of the measurement cell, thereby eliminating the need for a separate means for feeding the fluid sample to and from the measurement cell. It would also be desirable for such a device to be able to mechanically vary the optical path length during the process of making spectral measurements, thus allowing the measurement cell to be used on-line.